1. Technical Field
This invention relates methods and apparatus for measuring the thickness of a pipe wall in general, and to methods and apparatus for measuring the thickness of a pipe wall noninvasively using ultrasonic sensors in particular.
2. Background Information
A common problem for pipe lines of all types is the potential for corrosion and wear on the interior surfaces of the wall over time, and the potential for rupture of the pipe wall as a result of the degradation of the wall caused by corrosion and/or wear. The problem is particularly acute for applications that pass a fluid flow containing abrasive particulates through the pipe. The wall degradation typically cannot be visually detected because the interior of the pipe is closed, and contains the fluid flow. Stopping the fluid flow and dismantling the pipe to gain access to the pipe interior is not practical.
It is known to use ultrasonic sensors to evaluate wall thickness. U.S. Pat. No. 5,549,004 discloses a “Hand Held Tube Wall Thickness Ultrasonic Measurement Probe and System” (the '004 Patent). According to the '004 Patent, most ultrasonic testing is performed at frequencies between 1 MHz and 25 MHz, and the preferred frequency for the disclosure of the '004 Patent is 5 MHz.
One of the problems with a hand held wall thickness sensing device is that it is very difficult over a period of time to accurately monitor the exact same positions around the circumference of the pipe. Changes in the measurement position corrupt data collected over time. Consequently, it is problematic, and likely not possible, to accurately determine pipe wall thickness degradation trends. In addition, such hand measurement devices represent a cost to the party maintaining the piping system.
In many of the applications where it is desirable to monitor pipe wall thickness, it is also desirable to measure fluid flow characteristics within the pipe. Ultrasonic flow meters that utilize an array of ultrasonic sensors provide a particularly useful and robust tool for sensing flow characteristics such as velocity and volumetric rate of the entire flow, and for the constituents in those fluid flows having multiple phases (e.g., oil, water, gas). It is advantageous in such flow meters to utilize ultrasonic sensors that operate at frequencies below 1 MHz, which frequencies are below the frequency range utilized by commercially available ultrasonic wall thickness measurement devices.
What is needed is an apparatus and method for measuring pipe wall thicknesses that utilizes ultrasonic sensors, and one that can utilize existing sensors deployed within existing ultrasonic flow meters.